Generally, in case that a user equipment receives signals from a plurality of cells, misalignment of the signals received by the user equipment from a plurality of the cells may occur on time axis.
FIG. 1 is a diagram of signals received by a user equipment from two cells. Referring to FIG. 1, in case of OFDM based signal transmission and reception, a transmitting stage inserts a cyclic prefix in an OFDM symbol to prevent inter-symbol interference. In case that a plurality of cells are coordinated to transmit signals to one user equipment, even if discriminated signal waveforms are transmitted from a plurality of cells in a manner that cyclic prefixes overcome the misalignment of the signals of a plurality of the cells, the user equipment is able to successfully combine the signals received from a plurality of the cells.
FIG. 2 is a diagram for a case that a user equipment receives signals from 2 cells.
Referring to FIG. 2, in case that a user equipment receives signals from a cell A and a cell B, respectively, if a distance between the user equipment and the cell B is greater than that between the user equipment and the cell A, a propagation delay of the signal from the cell B is greater than that of the signal from the cell A.
Yet, when a user equipment receives signals from a plurality of cells, the user equipment is able to successfully decode the signals if the signals are almost simultaneously received by the user equipment from a plurality of the cells. In this case, the meaning of the almost simultaneous reception indicates that a timing difference between signals received from a plurality of cells should lie within a cyclic prefix.
FIG. 3 is a diagram for a case that a timing difference between signals received from a plurality of cells lies within a cyclic prefix.
Referring to FIG. 3, if a timing difference between a signal received from a cell A and a signal received from a cell B lies within a cyclic prefix, inter-symbol interference is not generated. Therefore, the signal from the cell A and the signal from the cell B can be errorlessly combined together.
FIG. 4 is a diagram for a case that a timing difference between signals received from a plurality of cells is greater than a cyclic prefix.
Referring to FIG. 4, if a timing difference between a signal received from a cell A and a signal received from a cell B is greater than a cyclic prefix, inter-symbol interference is generated from the signal received from the cell B because the signal of the cell B arrives after completion of the cyclic prefix of the signal of cell A. And, performance of system is degraded.